As the energy crisis continues, the use of coal gains in importance as an energy source. There is general agreement throughout the industry that tremendous coal reserves are available, leaving the more prominent problems of transportation and handling to be solved.
The equipment in which coal is burned has been steadily improved from the ancient locomotive where shovelfuls of the solid fuel were manually pitched onto a grate where the heat of combustion rose to the bottom of a water boiler. The particle size of coal is now controlled by sophisticated mills and is air-transported through conduits and discharged from nozzles into combustion zones. At present, development has concentrated on burning crushed coal in a bed supported by a horizontally extended perforated structure up through which combustion air is forced. The problems of introducing the crushed solid fuel into the bed fluidized above the perforated support are paralleled by the innovations in the system for removing solid residue from the combustion zone.
Beyond the problems of fuel flow into the bed, and ash removal from the bed, is the problem of inhibiting discharge of sulfur and nitrogen compounds with the gaseous products of combustion. There are many different approaches to the control of these substances which threaten the environment, and controlling the maximum temperature of the combusting fluidized bed is a significant factor. In general, the present problems revolve around maintaining a heat exchanger in contact with the upper portion of the bed to maintain control of its temperature.
Establishing the reaches of a tubular heat exchanger within the upper portion of the bed for temperature control, may appear to be a simple problem. The banks of heat exchanger tubes reaches are to be supported in their longitudinal extension within the bed. The banks of tubes may be horizontally spaced from each other at distances which will not interfere with fluidizing the combusting bed. The present problem is how to support these banks of horizontal reaches by a structure which extends up from the bed support plate. This support for the heat exchanger is exposed to the high temperature of the bed and, therefore, this temperature must be controlled to prevent thermal distortion and deterioration of the support. Further, the support structure must be arranged to provide plenary access for inspection and maintenance personnel to the volume above the support plate and below the supported heat exchanger. As the space between the heat exchanger and the support plate is opened up, the minimum number of drain ports will be needed for unburned residue.